There have been known a number of articles, such as a wrist watch, cuffs, an eye-glass frame, a tie-pin, a brooch, a stationery and other accessories or outer decorations, as well as a cap crown for a tooth or dental prosthetic in which a metal material is plated with a noble metal, such as gold. There have also been well-known in the art several methods for the purpose. These conventional articles, however, have a disadvantage in that the noble metal coating tends to come off and form specks in areas of frequent use, which adversely affect durability and appearance of the articles. Further, metal articles having on their surfaces lustrous patterns are also commercially available, which are, however, poor in durability and tend to come off by rubbing.
In order to overcome those disadvantages, there has been proposed an ion-plating method for forming a primary thin coating of hard metal compound on a surface of a metal material followed by a further ion-plating procedure for forming a secondary thin film of an elemental noble metal or an alloy having a similar color tone to that of the primary coating (GB Pat. No. 2000812A). The applicant has also developed a method of forming primary and secondary films over a whole area of a metal material or in a desired pattern, similarly an utilizing the ion-plating technique (UK Patent Application No. 8518159).
In general, a film-forming method utilizing the ion-plating technique comprises ionization of a portion of vaporized metal particles followed by deposition thereof on a solid surface to form a film, in which the ionization in the presence of an inert gas (such as argon or helium) provides a film of pure metal (elemental metal), whereas the ionization in the presence of an active gas (such as oxygen or nitrogen) provides a hard film of metal oxides or nitrides. Such procedure has been generally known as a conventional method for ion-plating in the art (Scientific Dictionary, Maruzen Co., Ltd., Page 53 (1985)).
GB Pat. No. 20000812A has also utilized the conventional method, as described in Examples, in which a primary film is formed in the presence of nitrogen as the active gas while a secondary film is formed in the presence of argon as the inert gas.
It has now been found out as a result of continued study of forming the two layered coatings of primary and secondary films by means of the ion-plating technique that the ion-plating procedure in the absence of the inert gas (such as argon or helium) during formation of the secondary film of elemental metal or its alloy differently from the conventional method may avoid operation, such as a high frequency output of argon gas pressure, and improve its operability. Further, a spattering phenomenon with an argon ion may be eliminated to increase a rate of forming a thickness of the film and to lower a treatment temperature, so that the material may be prevented from damage. In the absence of the inert gas (such as argon or helium), it has also been found out that the ionization may occur only with an evaporated metal to enable vacuum in an apparatus to be reduced to 10.sup.6 -10.sup.5 Torr which is lower by 10.sup.2 -10.sup.3 Torr than the order of 10.sup.8 Torr in the conventional method, to increase purity of the resulting film due to the absence of inclusion of, for example, the argon ion and to prevent pin holes, resulting in improvement of corrosion-resistance. Further, the enhanced ionization of the evaporated metal permits working in a place of lower voltage applied to a cathode, thereby to reduce troubles due to abnormal discharge.
Accordingly, an object of the invention is to provide a method of processing a metal surface by means of an ionplating technique, which is superior in operability, economy and properties of products to conventional ion-plating methods when a hard primary metal film is formed on a surface of a metal material followed by a secondary film of elemental metal or its alloy thereon.